Charakterisierung der Nichtwirt-Interaktion zwischen Gerste und Pilzen der Gattung Magnaporthe

  • Characterisation of the nonhost-interaction of barley and fungi of the genus Magnaporthe

Zellerhoff, Nina; Slusarenko, Alan (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2009, 2010)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2009

Abstract

Nonhost-resistance (NHR) is the resistance shown by a plant species towards all isolates of a pathogen species. This durable type of resistance protects plants against the majority of potential pathogens. In this thesis the nonhost-interaction between barley and isolates of the fungal genus Magnaporthe was analysed and compared to the host-interaction. Magnaporthe was isolated from different host plants, and the different isolates were examined for their biological relationship to one another and for their capacity to infect barley. It was shown that all isolates which were able to cause disease symptoms on barley (host-interaction) belong to the species M. oryzae. In contrast, M. grisea isolates (host: Digitaria) and isolates of a putative novel species which is associated with the host Pennisetum could not reproduce on barley (nonhost-interaction). Comparative cytological analyses showed that formation of papillae (penetration resistance) and hypersensitive epidermal cell death (postpenetration resistance) were predominant nonhost-defense mechanisms. These reactions were apparent by 24 h p.i. at higher frequencies in the nonhost-interaction than in the host-interaction. It was possible to significantly reduce the penetration resistance of barley by treatments with AIP (an inhibitor of phenylalanine ammonia lyase) and cytochalasin E (an inhibitor of actin-polymerisation). Similarly, penetration resistance was reduced in the barley mutant mlo-5 ror1-2 rar1-2. However, growth of the nonhost-pathogen was arrested in epidermal cells in all cases and the pathogen was never able to generate secondary hyphae. These results underpin the effectiveness of epidermal defence in this nonhost-interaction. Complementary-DNA macroarray-analysis of inoculated barley-epidermis showed a very early (6 h p.i.) and pronounced change in the pattern of gene-expression in the nonhost-interaction as compared to the host-interaction. Particularly, the expression of genes involved in lipid metabolism was induced significantly after inoculation with the nonhost-pathogen. For functional analysis of these candidate genes virus-induced gene silencing technology (VIGS) was established for the barley/Magnaporthe-interaction. Results presented here indicate similarities between the NHR and basal resistance of barley towards fungi of the genus Magnaporthe. In general, barley responds to pathogen attack with a similar spectrum of defence mechanisms in nonhost- or host-interactions, but activation of these defences occurs earlier and more efficiently in the nonhost- than in the host-interaction. This phenomenon might depend on an early recognition of nonhost-specific effector molecules by barley. In contrast, the suppression of plant defence by effectors of host-pathogens might be responsible for the delayed defence in the host-interaction. Remarkably, a comparative transcriptome-analysis of three different nonhost-interactions of barley (barley/Magnaporthe, barley/powdery mildew, barley/rust) revealed no genes which were regulated in a similar manner in all interactions. These results point to a pathogen-specific reprogramming of the plant metabolism in nonhost-interactions. During further investigations the significance of the Rom1 gene for the interaction of barley with different species of the genus Magnaporthe was analysed. The mutation in Rom1 restores the resistance of susceptible Mla12 rar1-2 Rom1 (rar1-2) barley against the powdery mildew fungus Bgh. Remarkably, after infection with M. oryzae, plants of the genotype Mla12 rar1-2 rom1 (rom1) showed more but smaller disease symptoms than wildtype and rar1-2-plants. Thus, Rom1 and Rar1 appear to act antagonistically in epidermal and mesophyll defence scenarios. Moreover, rom1-plants showed a higher accumulation of PR1b-specific transcripts after inoculation with the Magnaporthe nonhost-pathogen in comparison to wildtype and rar1-2-plants. This may argue for an augmented responsiveness of rom1-plants. Furthermore, the influence of the gene Rac1, whose overexpression is known to cause hypersusceptibility of barley to Bgh, was investigated in the barley/M. oryzae interaction. In macro- and microscopical analyses a correlation between increased resistance of Rac1-overexpression lines against M. oryzae and augmented penetration resistance was observed. These results point to an influence of Rom1 and Rac1 on basal resistance and, in the case of Rom1, on the NHR of barley against different pathogens.

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